Treatment of cellulosic materials



Jan. 10, 1961 L. REINHARDT ET AL TREATMENT OF CELLULOSIC MATERIALS Filed Nov. 6. 1956 3' D *FH G3 YX- X K r-g q,

United States Patent O TREATMENT or cELLULosIc MATERIALS Lothar Reinhardt, Aschau, Kreis Muhldorf, Upper Bavaria, and Friedrich Stecher, Aschau, Kreis Juhldorf, Upper Bavaria, Germany, assignors to Wasag-Chemie A.G., Essen, Germany Filed Nov. 6, 19156, Ser. No. 620,662

Claims priority, application Germany Nov. 7, 1955 1 Claim. (Cl. 23-285) The present invention relates to a method and device for the treatment of cellulosic materials, and more particularly it relates to a method and device applicable in the production of cellulose compounds, particularly cellulose esters.

It is well known to use as raw material for the production of cellulose derivatives such as cellulose nitrate or cellulose acetate, either cotton linters or wood cellulose.

In order to obtain uniform esteritication of the cellulosic fibrous raw material, it is necessary to completely loosen the fibrous material so as to disintegrate any fiber agglomerations therein and to obtain a mass of individual free fibers.

Ideally, when the material which is subjected to esterification should consist only of individual cellulosic fibers which are completely separated from each other.

The above mentioned .raw materials which are practically exclusively used for the production of cellulose derivatives, namely cotton linters and wood cellulose or wood pulp, do not fulfill the above stated ideal requirement of being composed exclusively of individual free fibers. According to customary methods cotton linters are obtained in the form of loosened fibrous mass, however, this mass contains always a larger or smaller quantity of small felt-like or even hard knots which cannot at all or only diflicultly be penetrated by the esterifying liquid. Up to now it has not been possible to practically completely loosen or eliminate these knots. Far more than in the case of cotton linters, difficulties are encountered in the esterification of cellulosic mate-l rial derived from wood pulp. Such materials are usually in the form of paper or cardboard. In these products, the individual fibers are not separated from each other but are felted or glued together. It is then necessary to so treat the cellulosic materials such as paper or cardboard as to make each individual fiber accessible for penetration by the esterifying liquid. Only in this manner would it be possible to obtain a uniformly esterilied final product. In order to achieve optimum separation of the individual fibers, cellulosic material is frequently transformed into thin crepe paper which can be penetrated by the esterifying liquid with relatively little difficulty. However, the production of crepe paper as a preliminary step to the esterifcation of cellulose is expensive and frequently would increase the costs of the entire process to an uneconomical level.

Consequently it has been tried to use low cost pulp sheets or cardboards as starting material for the production of cellulose, esters.v Various attempts have been made to overcome the difiiculties inherent in the use of raw materials of this type. It has for instance been attempted to nitrify such sheets directly with or without first comminuting the same or cutting the same into rough pieces. In these cases it was then attempted to obtain the desired degreecf esterification by carrying out the esterification process under specially designed conditions different from established .standards and with ICC varying mixed acid compositions. It has also been tried to cut dry pulp sheets into small squares of about 1.5 mm. length. In this case too it is necessary for producing nitro-cellulose to employ a mixed acid of abnormal composition, in this case containing an especially high percentage of nitric acid. It is the great disadvantage of this method that very costly regeneration installations are required for working up of the waste acids of the process.

It has also been proposed to scrape by means of a special machine very thin slices olf pulp sheets, which slices are to be sufficiently thin to allow complete penetration thereof by the esterifying liquid such as nitrating acid. The machinery required for this purpose is not only very expensive but also requires constant costly maintenance.

Furthermore great efforts have been made to construct the machinery capable of transforming the low cost pulp sheets into a suiciently loosened fibrous mass by mechanical means. Various types of comminuting machinesV y Itis therefore anV object of the present invention tov i provide'a method and apparatus capable of overcoming as `final suspension of bleached cellulose prior to further s the aforementioned difiiculties in the esterification of cellulosic fibrous raw materials.

It is a further object of the present invention to provide a method and apparatus for uniform esterification of cotton linters, wood pulp sheets and the like.

-It is yet another object of the present invention to provide a method and apparatus for the uniform esterification of cellulosic fibrous raw materials in an easy and economical manner.

Other objects and advantages of the present invention will become apparent from a further reading of the description and of the appended claim.

With the above and other objects in view, the present invention comprises in a method of treating cellulosic fibrous materials the steps of forming an aqueous suspension of fiber agglomerations containing cellulosic material; and defibering said suspended material, whereby `an aqueous suspension of substantially free cellulose fibers is formed.

Ille present invention also contemplates in a device of the character` described, in combination, defibration means for separating fiber agglomerations contained in an aqueous suspension of cellulosic material into individual cellulosic fibers, dehydration means for at least partially dehydrating a suspension of individual cellulosic fibers formed in the defibration means, and conduit means operatively connected to the defibration means and to the dehydration means for passing a suspension of Y in the case of wood cellulose may be a suspension such` as is obtained in the production of cellulose from4 wood sa-Me treatment of the same in a paper or cardboard maker. In the case of cotton linters, the starting material according to the present invention is represented by the linter suspension which is obtained as final product during the preparation of cotton and consists of cleaned and, if desired, bleached linter fibers prior to dehydration Aand drying of the same.

The aqueous suspension of cellulosic fibrous material still containing fiber agglomerations, such as knots, feltlike particles and the like is first introduced into a device in which complete loosening and debration of all fiber agglomerations is achieved. Such fiber agglomerations consist primarily of knots and felt-like particles when a suspension of cotton linters is used, while cellulose suspensions which were obtained by disintegration of pulp sheets and the like contain hardened particles which offer great resistance against complete detibration and separation into individual fibers. All these agglomerations are completely disintegrated into individual fibers so that the aqueous suspension leaving the delibration device contains a homogeneous distribution of individual free and separated fibers in water. It has been found according to the present invention that the defibration device preferably is so constructed as to be free of or including only a relatively minor fraction of milling forces acting on the suspension. Essentially, the forces generated in the deibrating device and acting upon the fiber agglomerations therein are forces such as hydrodynamic pressure waves, cavitational forces Vcreating repeated pressure fluctuations and partial vacua, ultrasonic waves and the like. A device which may be advantageously used as defibrator according to the present invention is known under the trade name Supratonatorf and has been described in detail in Chemie-Ingenieur-Technik 1952, p. 249/250; 1954, p. 122/123. This apparatus essentially consists in ne'or more. throttling passages in the form of a plurality of chambers and'located in'the" path of fiow of the material which is to be treated. In these throttling passages hydrodynamic pressure waves are mechanically produced in the suspension at such frequencies as result from the speed of circulation and the number of chambers.

The suspension of free fibers leaving the defbrator has normally a stock density of about 7%. This suspension is now dehydrated preferably by means of a con tinuously operating screw'centrifuge, whereby the fiber mass which is fiung ofi onto an outer conical screening drum, is removed by means of an inner conical screwA drum. This is accomplished in suenav manner ythat the rate of revolutions of the two drums differs fromeach other so that the number of revolutions per minute ofthe inner conical screw drum is about less than the number of revolutions per minute of the outer conical screening drum. A centrifuge of this type is for instance described in Ullmanns Enzyklopaedie der technischen Chemie 1951, vol I, page 515, and illustrated in Fig. 512. The partially dehydrated cellulose fibers leaving the centrifuge have a moisture content of about 20-50%, depending on the rates of rotational speed of the drums, and are free of agglomerations. The fiber mass is of completely loose structure inv which the individual fibers are free and movably separate from each other. Surprisingly it has been found that the treatment of the suspension in the centrifuge does not only cause a reduction in the Water content so as to form al'fibrous'xnass of between 50 and 80% dry substance,butthat'simul? taneously a purification of the celluloseY fibers is Vaccomplished inasmuch as difiicultly esterir'able impurities which are present in normal cellulose, as well as ditiicultly esterifiable short fibers medullary cells containing particles, pass with the centrifuged water through the 'open ings in the outer conical screening drum, and are thereby removed. A similar advantageous effect can also` be observed during the centrifugal dehydration ofvcotton` linter suspensions, inasmuch as thereby 4the very ,short and dust forming fibers are removed together with `the centrifuged water. Removal of these very short fibers is important because the same tend to lump formation when introduced into the esterication liquid. Furthermore, investment and operating expenses are reduced by employing the above described centrifuge since about 70% of the water content of the suspension are mechanically removed and consequently the costly customary drying and dehydration machinery generally used in the cellulose industry will not be required.

After drying the centrifuged mass of free cellulose fibers in one of the dryers conventionally used in the industry, a completely loose mechanically purified mass of individual fibers is obtained, which mass upon esterification, contrary to the results obtainable with the method customary in the industry, is converted into a uniformly esterified soluble product. If desired, it is also possible to perform the individual steps of the above described process at different locations. For instance, treatment of the aqueous suspension of fibrous cellulose material in the defibration device and in the screw centrifugal machine as well as subsequent drying of the fiber material, may be carried out in the pulp mill and cellulose plant. In case the cellulose plant is not equipped with a suitable drying apparatus, it is also possible to compress the moist fiber material in the condition in which it leaves the screw centrifugal machine, into bales which may be shipped to the esterification plant where loosening of the bales and subsequent drying of the loose ber mass may be carried out with conventional equipment.

It must be noted that the terms debration and to defibrate are used in the present specification to denote the Aseparation of fiber agglomerations, fiber bundles, knots or felted particles of fibrous material, and the like into individual fibers freely movable relative to each other. The present invention is applicable to various cellulosic fibers which depending on their origin, may ormay not have been pretreated for instance for removal of lignin, by bleaching or in other manners in order to remove at least part of the non-cellulose material adhering to the fibrous material, and which cellulosic fibers, untreated or after such treatment still in part adhere to each other, form agglomerations, knots, etc., and thus do not lend themselves to completely uniform esterifcation, as has been described further above.

The process of the present invention may also be carried out with dry or moist cellulosic boards such as are produced in cellulose plants. Such pulp boards or cardboards are then first beaten in water so as to form a suspension thereof, and the thus formed suspension is then further treated according to the present invention in a defibrating device such as the Supratonaton subsequently passed through a screw centrifugal machine, and finally dried. Whether it is more economical to carry out the entire process and the esterification at one location, or to transport an intermediary product to a different location for further processing, depends on local conditions, freight rates and the like. Preferably cotton linter suspensions are completely processed in the cotton bleaching plant. According to another embodiment of the present invention, the debration of the fibrous mass in a Supratonator takes place simultaneously with the esterification of the cellulose. Hereby roughly comminuted cellulosic fibrous material is introduced into the esterification liquid such as nitrating acid.. The reaction mixture is'then pumped through the Supratonator whereby only superficially gelatinized particles the core of which has not yet been wetted by the nitrating acid or the like, are completely defibrated so that the thus separated individual fibers can then easily be attacked' by the esterifying liquid. Cellulose esters such as nitrocellulose which are produced according to the method of the present invention, are distinguished by the fact that the otherwise present unfilterable swollen gelatinized particles are absent, so that the entire cellulose ester solution can be more easily filtered;

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

=Fig. 1 is a schematic illustration of an installation according to the present invention for the esterification of cellulose boards; and

Fig. 2 is a schematic illustration of an installation according to the present invention for the esterification simultaneous with the debration of fiber agglomerations.

Referring now to the drawing and particularly to Fig. l, a conventional tearing and comminuting device 1 is shown in which dry cellulose boards in sheet or roll form are roughly comminuted. The thus comminuted material is introduced into mixer 2 together with a sufficient quantity of water to form therein a pumpable pulp containing up to about 7% solid material. In order to transform this pulp into a suspension of individual free fibers, the pulp passes through defibrator or defibration means 3, for instance through a Supratonator. Dehydration of the homogenized pulp consisting of a suspension of free fibers in water then takes place in dehydration means 4, preferably a screw centrifuge. The cellulose fibers leaving screw centrifuge 4 pass then into the receiving portion of dryer 5, preferably a forced air dryer. When fiber separation is to be carried out at the same location at which the cellulosic material has been produced from wood, cotton or the like, the cellulose pulp produced in such pretreatment is directly introduced into mixer 2 and from there on treated as described above. The water which is separated from the fiber suspension in screw centrifuge 4 contains small fiber particles, dust-like particles, as well as medullary cells. These particles which remain suspended in the water which is separated from the fiber mass in dehydration means 4, and which usually amounts to between 3 and 10% of the initially introduced cellulosic material, are then filtered out in a cell filter 6 or a similar device and can be used for the production of low quality cellulose products such as low quality paper. The clean water leaving filter 6 may then be re-introduced into the process, for instance into mixer 2.

Referring now to Fig. 2 of the drawing, dry cellulose board or cotton linters are first passed through a conventional tearing and shredding device 7 such as a hammer mill, and are then passed in roughly comminuted state into dosing and measuring device 8. From there the cellulosic material drops into a first reactor 9 into which nitrating acid is simultaneously introduced. In this manner for instance 20 kilograms of cellulose are mixed with 1000 kilograms of nitrating acid. Reactor 9 is built as a vessel of circular or oval cross-section and is provided with stirrers. Other types of reactor vessels may however also be used, such as reactors in the shape of an elongated trough through which the nitrating acid fiows. Within such trough a perforated cylinder is arranged through which the mass is conveyed by means of a screw conveyer. This latter construction of the reactor vessel 9 is preferred when it is desired to operate in a continuous manner rather than in separate batches. After the mass has been penetrated by the acid, the mixture is passed to Supratonator 10 for homogenization and separation of the rough particles in the mixture into separate fibers. From the defibration means 10, the mass is pumped into second reactor 11 in which the mixture remains until substantially perfect esterification as desired has been achieved. This usually requires 10 minutes or more. In case of discontinuous batch-like operation several second reactors 11 are provided which are successively emptied into a centrifuge when esterification in each of the second reactors is completed. However, the second reactor 11 may also be designed for continuous operation similarly to first reactor 9. For

deacidification of the esteried mixture, preferably a.A

continuously operating centrifuge of suitable design (not shown in the drawing) is provided.

The following examples are given as illustrative only of the method of the present invention, the invention however not being limited to the specific details of the examples.

Example 1 An aqueous suspension of 100 kilograms spruce cellulose having an a cellulose content of about the fiber density being about 6%, is passed through a defbrator of the Supratonator type and is subsequently dehydrated in a screw centrifuge of the type described earlier in the specification. The conical sieve drum of the screw centrifuge has a diameter of 350/ 650 millimeters and revolves at 2,200 revolutions per minute. The inner screw drum revolves at 2,000 revolutions per minute. The dehydration requires about 15 minutes. The fibrous cellulose mass leaving the centrifuge is of entirely loose structure and contains 70% dry substance. The yieldA amounts to 98% of the original cellulose quantity. The loosened fiber mass is then dried in a forced air dryer such as a Buehler dryer to a moisture content of between l and 2%. The mass is then nitrated in customary manner, and a collodion cotton is obtained employed in the lacquer industry.

Example 2 100 kilograms of dry beech cellulose boards having an a content of 91%, are suspended by beating in about 2,000 kilograms water. In order to defibrate remaining fiber bundles or other coarse fibrous particles, the suspension passes first through a Supratonator and then through a screw centrifuge. The cellulose mass leaving the screw centrifuge is a loose mass of separated fibers and has a moisture content of about 30%. All of the originally present fiber bundles, agglomeration and the like, have disappeared. After subsequent drying to a moisture content of 2%, the fiber mass is tested with respect to its suitability for esterification, specifically acetylation, according to the method described in Chemiker Zeitung, issue 9, page (1952). Clearness and filterability of the reaction mixture containing cellulose triacetate are excellent and similar to clearness and filterability of a solution made under similar conditions from the best linter triacetate.

Example 3 An aqueous suspension of 1,000 kilograms of bleached linters having a fiber density of about 8% is treated for a short period of time with the defibration means described in Examples l and 2, and is subsequently partially dehydrated in a screw centrifuge. A linter fiber mass is obtained in this manner which is practically free of napes, knots or any other fiber agglomeration, and which after drying is outstandingly suitable for all kinds of esterification reactions such as the production of nitrocellulose, acetyl cellulose, aceto butyrate, which esters form clearer solutions than esters prepared from linters which were treated in customary manner, i.e. without complete fiber* separation according to the present invention.

Example 4 It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types` of treatment apparatus for cellulosic materials differing from the types described above.

While the invention has been illustrated and described as embodied in a device for the treatment of cellulose obtained from wood pulp and cotton linters, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claim.

What is claimed as new and desired to be secured by Letters Patent is:

In an apparatus for forming nitric acid esters of cellulose from nitrating acid and a cellulosic material containing cellulose bers and cellulose fiber agglomerations, in combination, mixing means for mixing nitrating acid and cellulosic material; pressure uctuations creating means operatively connected to said mixing means for subjecting the mixture of nitrating acid and cellulosic material to hydrodynamic pressure waves of sucient intensity to cause substantially complete disintegration of the ber agglomerations of said raw material into individual bers and for simultaneously partially esterifying the same, said pressure fluctuations creating means comprising at least one throttling passage means for passing said mixture therethrough and for mechanically producing in said mixture hydrodynamic pressure waves whereby the frequency of the thus produced pressure waves may be controlled by the speed of circulation of said mixture and the number of throttling passages in said passage means; and means for completing esterication of the thus partially esteried bers operatively con nected to said pressure fluctuations creating means.

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